It cost 60 million IDR (divide by ~ 400 to get THB, so around 150,000 THB), and measures around 8.5 metres * 4 metres. It backs onto a rock.

Plans were for an quick refurb/cheap build as an office for my travel company

So in November 2015 I went from England to Sumatra for a month to refurb.

Upon starting the refurb, the owner of the rock behind wanted to sell it to me. This also included a strip of adjacent land. This was another 60 million. I decided that the quick and simple shack with a simple zinc roof idea would be cancelled, and we would go for two floors with a flat roof as a two-storey building with flat roof would work well as we could then use the flat roof and the much larger rock as a sitting area/bar area/BBQ area/whatever.

So my wife, who is clueless about building, and was in England, insisted that I build this whole thing in a month (!). I told her we hadn't found any builder. She has infinite, but deeply misplaced, confidence in her mother, who procured a group of workers. The lead builder was short and fat, and did not know what he was doing.

However, in order to settle things down, as one of the neighbours seemed to be in some confusion about where the boundary with his land was, I built the piers at the border of the land in order to settle the issue. Unfortunately the builder did not know much about how to build reinforced concrete (and neither did I), and the concrete was soft, and the rebar not covered. In addition, when we decided that we would not build a one-storey shack, but rather a two-storey building, the rebar footings (known here as chicken feet) he tried to expand by tying some more steel on to them, a completely futile exercise. The concrete was not mixed according to any standard proportions, and rather than using sand and crushed stone as aggregate as advised by another Westerner who knew what he was doing, he simply ignored advice (even though he was on a daily rate) and used a cheaper product known as sirtu (apparently short for pasir batu, or sandy stones).

After a month I went back to England, but it was clear that essentially everything we had built would have to be knocked down.

So I returned again in January. Before departing at the beginning of December I had found a builder, who was the brother of my 'adoptive' (according to local tradition to enter the tribe) sister's husband. He made great promises about his capabilities and things were quite promising. I suggested that he met me at the airport, and that we would then drive through the city to find steel, tools, etc. He did not seem very keen on this, and in the end we ended meeting him in his village, some three hours distant, which turned out to be an alarmingly backward looking place, and he brought his crew of scrawny looking staff who apparently had few or no possessions. This was again not a good sign.

We set out to find some rebar - having previously during the abortive construction attempt in November, constructed with smooth steel of 8 and 12mm we had resolved instead to use much stronger 16mm and 12mm deformed steel. Again, alarmingly, he did not really seem to know where to go, but eventually we found a store selling rebar, which cost at that time 123,000 IDR and 67,000 IDR for 16mm and 12mm respectively, and we ordered (they had no stock in place) 100 bars (of 12m length) of each.

We also rented a jackhammer, which cost us 275,000 IDR per day. This was intended to destroy the old concrete and also some rock, in order to build the piers. In the end, we ended up keeping the jackhammer for eight days (including essentially two days for return transport), as there was frequent all-day power outages, apparently as a result of trees which local farmers selfishly plant beneath power lines, needing to be trimmed and therefore the power shut off. The jack hammer was some years old, and it delivered an electric shock to users, as there was a loose contact inside, which we repaired. In addition, it seemed that rather than an original Hitachi or Bosch bit, they had provided some cheap, soft Chinese bit, which cost 1/4 of the price, but was completely incapable of destroying our (limestone) rock.

I had also resolved to source a cement mixer (readymix is not an option here), in order to improve concrete quality, however the locals were very keen on noisy diesel models, and it was very difficult to find a lightweight (150l) electric model. Eventually one was sourced, from Jakarta, which took more than a week to arrive, and cost around 5 million.

Having chosen an electric cement mixer, it was necessary buy a generator due to the guarantee of daily powercuts. A quick survey of the market revealed that you get get some Chinese-branded junk for around 2 million, or a 2000W Honda or Yamaha conventional petrol for around 6 million. Meanwhile there was almost no market for inverter models, as these would cost close to 20 million for the same capacity. Rather than face the risk of having a sleak inverter generator being stolen, or suffer with Chinese junk, I went for the Yamaha, as it claimed smoother power (finding test results is nearly impossible here). The showroom promised that the first oil fill was free, however when it arrived and we tried to start it, it was apparent that they had not in fact put any oil in the machine. Fortunately there was a safety cut-off, so the generator was not ruined.

Although our original builder from November had assured me that the septic tank had been checked and was 'ok', it was apparent that this was not the case. We had been clearing rubbish from the back of the plot, where neighbours had dumped it over several years, using a grandfather from the neighbouring village and his pickup, plus two well-built workers, apparently his neighbours. We asked the pickup man to dig us out a septic tank. This was done for the sum of 2.1 million and took four days for two people, including dumping the earth removed.

Meanwhile, it was increasingly apparent that not all was quite right with January's builder and his staff, one of whom was very petulant and did not seem to be keen on working. As I was paying not only wages, but also food and board for these staff, this was not very impressive. Some of them seemed to be friendly with some of the more dubious of the neighbours, and there were suspicions that their wages were being spent on methamphetamine, hence the attitude. Things came to a head, and after words were exchanged, the labourers sloped off back to their village in a huff. At the weekend, the main builder also disappeared, without saying a word, and did not return for several days, having brought with him his brother's motorbike, and also having borrowed a small sum of my money. He returned several days later and was thereby terminated.

Our septic tank diggers, on the other hand, were strong and fit, unlike the slight labourers brought by the builder - the thinking here seems to be that when you have a daily rate, you bring your cousin, or your son, or some other such nepotism, and when you are paid by the job, you bring the strongest/quickest worker you can find. Hence, the septic tank diggers, having been sourced for piece work were strong as a pair of oxen. So after the septic tank was done, we asked for them to stay on a daily rate, which they did.

Having had our main builder disappear, we were left with his brother (my adoptive sister's husband). It became quickly apparent that he was not up to the job. He was also aware of this, and tried to find some help. However the builder he offered made it apparent that he would leave the site as soon as he got another offer, just leaving a worker behind.

So we tried to find a new builder, which we did via the friend of a friend's builder. He measured up the three piers that had been built by the existing builder and pointed out that they were not in a straight line, and moreover that they did not follow the line of the road. So we would have to demolish at least one of the three, in order to get a straight line. Having made this resolution, the builder's brother would have to be sacked. The next morning, I planned to head to his house before work (8am) to deliver the news. However, he knocked on my door at 7am, wanting to borrow money. I told him there and then that he was finished, as we had to demolish his work there was no way he could keep going. This caused quite a scene, as his wife (my 'adoptive' sister) shouted racist abuse at me and some of her family also joined in. Here it is not really the done thing to sack people, it is preferable to use an excuse so there is no loss of face.

The new builder promised that he had a friend who, unlike him a village worker building simple village houses, had experience in the city, building multistorey hotels and such like. This turned out to be true, but he did not arrive for several weeks. In the meantime he was on his own. Although he was more capable than the previous two builders, he is not all that tidy in his work, and in addition the task of considering the various plumbing/piping works were quite beyond him. (Experience has suggested to me that while some local knowledge might be correct, much is not, and you should not trust any of it.) In the end, we installed 4" pipes for the toilet, a 1" vented pipe for the septic tank, a 2" pipe (which should have actually been 3", to admit the passage of 2 * 1" input pipes) for water from the river, and 3" pipes embedded in all of the piers to drain rainwater, which are then joined into a 4" pipe at ground level, and then into the river. The practice of embedding pipes in piers is one which does reduce the strength of the column - although with our 12" square piers (even thicker in reality - as I said, the builders are not exactly tidy) not by too much. We also installed a six-inch pipe, in theory to empty the septic tank (in practice there are no septic tank drainage contractors here, though we might be able to find one), which is buried underneath the floor, and then a 1" pipe for fresh water tankers. In theory we would dig a ground water wall, however due to the hard rock, nobody in our immediate area has one, and it is not clear where we could find a contractor. Therefore, while the river water is clean, during heavy rains there are often landslips resulting in the water turning brown making the river water unusable for several days (hence the need for trucked in water).

In addition, during the process of fitting pipes, considering the flow (gravity!) of solids and liquids between the three chambers of the septic tank, from the first septic tank to the second seemed to be a task quite beyond him. Fortunately my friend was able to pass comment, otherwise the 6" pipe would have been permanently full....

The new builder, who had spent some years in the city, turned out to be much more capable than his friend, and had experience with projects run by foreign architects, multi-storey hotels, concrete slabs (without ring beam, and so on). He is however not as good-humoured as his friend, and there appears to be a slight culture clash in that he is a fairly devout Muslim, more so than than the other workers, and at one point he disappeared for several days for reasons not entirely clear, and may not have actually returned to the site had I not refused to pay his wages other than in person.

The village has several independent builder's merchants, however they are poorly stocked and for anything more complicated than stirrup rebar or cement, it's a 3 hour drive, unless you are willing to settle for cheap and nasty. Although I had considered using AAC blocks and panels, it appeared that the international brands were not available in Sumatra. However, on a trip to the city where I trawled around the large air-conditioned builders merchants, which boast (for example) more than one brand of paint, more than one kind of glass block, and granite tiles, I happened to see AAC block in person, however at a fairly steep price. We were at the point where we had erected our concrete piers and the builders wanted to infill walls, although as I understand it is better to build the roof before the walls, so that the roof can deform in place rather than also compressing the walls.

However, being impressed by the AAC blocks I had seen, I tried again to source some from a wholesale. After making enquiries, I found a source in the city, and I rented a truck (six cubic metre capacity) to drive there. This journey took around six hours (instead of three in a car), as some bridges had height restrictions, and the driver tried to avoid them, but ended up reaching more height restrictions. After six hours I looked at the AAC blocks, and was impressed. We paid 1.155 million IDR per cubic metre, for 60x20x10cm blocks, plus 107,000 IDR for thin bed mortar, for a 40kg sack. We are told 1 sack per cubic metre, but this was not sufficient.

The wholesaler did not have any of the notched AAC trowels in stock, so I got them to drop me off at one of the air-conditioned builder's merchants. Here I picked up a trowel for around 75,000 IDR. I also browsed cement additives, door hardware, and glass blocks, which I would use for a bathroom window (as an alternative to clear glass), and for decoration in a storeroom. For a 1.5 metre two-leaf sliding door, hinges, rails, brackets and other fittings (excluding locks and cylinders) ran to 1.3 million rupiah.

In the event, the workers did not like the notched trowels, and preferred to use a regular trowel for the thin-bed mortar. After our first six cubic metre load was finished, I sent the driver back for a second. This load, however, had been outside in the rain, perhaps for months, and more of the blocks were damaged or broken. After this, I decided to investigate locally produced AAC blocks, as our blocks had a higher price than where they were produced, Jakarta, due to transport costs. I obtained a sample of block and mortar, and we tested them out. The locally produced mortar, while cheaper, seemed very weak, but the blocks, considerably cheaper at 825,000 IDR per buic, were better than the Javanese made blocks that had been in the rain. So we sourced the mortar from the original supplier (the mortar was much cheaper there than from the builder's merchant, however the brands are different - some Googling suggests that there is a standard for compressive strength for thin bed mortar, so clearly not all are equal), the driver having to collect the sacks in a becak (tuktuk) to avoid them knowing we had purchased our blocks elsewhere, and the blocks from the new supplier.

I had ordered a rather elaborate set of front door and window frames from a local carpenter. The frames were separated into head and body, and the heads, being round, was beyond the capabilities of our carpenter, not-to-mention expensive, and he had them made in a somewhat distant town. This caused some delay, as the frames would need to be ready before the rest of the wall. Eventually they arrived, and my workers spent several days sanding by hand. After that I bought a sander, which cost around 750,000 IDR for a Maktec (Makita) brand. Locally people like to paint wood in gaudy colours, however I used a varnish. Although there are water-based varnishes available, the city's builders' merchants claimed not to sell them, so I went with oil-based. My frame head was made out of damar (agathis wood), however after ordering, I read up on tropical woods, via Google and other source, and it appeared that damar is a conifer that is not termite resistant, nor resistant to water, and the premium choice, merbau, which the locals are a bit wary of, as it makes spooky noises as it expands and contracts in the heat, would have been better. So the body was made from merbau, which should last 100 years, whereas the damar unfortunately will not. The damar is a yellowy colour and merbau reddy-brown. I went for a varnish they call 'solar yellow', and tried varnishing the damar in 'solar red' to try and blend in, however this was a mistake that we sanded down, as it was too vibrant. I also made a mistake with wood filler - I purchased wood putty, which is white, and can be painted but not stained (varnished). This was also sanded. Subsequently, I purchased 'merbau' coloured wood filler, there being no damar-coloured wood filler, however the merbau filler also does not blend very well with the yellow damar.

Sorry about the lack of pictures, internet here is very bad. I'm just uploading now to Sharepoint, will see how that goes, but have got around 17.5GB to go, so I guess it could be a week or so.

Anyway, we are getting ready to pour our first floor; Eid-ul-Fitr starts on Wednesday so it's essential to get the concrete down before then.

I went out concrete additive shopping, to look for something to make the concrete hard quicker (readymix is NOT an option here, although you can get it in the city). This was somewhat fruitless, as they only had 900ml Sika at 35,000 IDR, and 5kg of generic brand at 100,000IDR. I didn't want the generic brand, and the Sika didn't seem very economical.

So I went home (3 hours drive) with no concrete additive, but then did some research. It seems that I was looking for a type F concrete admixture, that is a high ranger water reducer and superplasticizer.

Further research suggests that there are three main types:

Formaldehyde-sulphonate- this is the cheapest, and runs around 25,000 IDR per litrePolycarboxylate - this costs around 50,000 IDR per litrepolycarboxylate ester - this is not generally in the market here yet - not only this, but many of Sika Indonesia's products are old technology that have been superseded in the USA. BASF apparently make a product called Master Ease, but I'v e not seen their products locally.

According to this study http://www.revmaterialeplastice.ro/pdf/ ... 1%2012.pdf, Polycarboxylate at 0.6% of cement weight doubles the concrete strength, and reduces water penetration by 75%. Polycarboxylate ester at around half that concentration (0.3%) achieves the same result. Meanwhile formaldehyde sulphonate cannot match this, and for good improvement of strength, 1% is needed. So there is no real economy from buying the cheaper product. However, the strength is predicated on a 40% reduction in water usage, which is essential for the builders to follow, otherwise the money is wasted.

I have calculated around 13 cubic metres of concrete, and have ordered 125 sacks (5 tonnes) of cement, based on a 1:2:3 (cement:sand:crushed stone) ratio. Therefore, a cubic metre of concrete costs me roughly:

Although in theory I think it works out a little cheaper to use a plasticizer and less cement than to use more cement and no plasticizer, nobody here uses plasticizer. Probably because most concrete is hand mixed and they add too much water, and don't follow any kind of proper recipe.

One tip is to buy many more of the standard builders buckets than you think you will need, keep a secret supply so you can replace broken ones without having to make another buying trip.To have any chance of them putting in measured amounts it will have to be by the bucket, and to get them to use the buckets and not guess you will need 2 or 4 times the number for any one mix. Then you have a chance of them filling the buckets while the current mix is going and using the buckets to fill your mixer.

Sometimewoodworker wrote:Good luck with getting your workers to follow any kind of mixture.

I totally agree with STWW. I had a site manager for the first month and he had to stand next to the 1 cube mixer and piss-off the workers trying to cut corners by putting in a bag of cement and then just throwing in sand and gravel to their specs.A metal bucket if you can get one is the way to go. Do not let them use their plastic "measuring" devices. I think you'll have your work cutout, by the sound of it, Indonesia is far worse than Thailand.I wish you good luck.

This beam is at the front only, over the doors/windows, and is 54cm deep, and of 4 * 16mm deformed steel at the corners, with 12mm steel in the middle (this was added by the builders - my online Indian structural engineer specified much less steel than was actually used)

12mm steel, top and bottom @ 20cm, and @ 10cm within 90cm of each column. Note that this was specified by the rent-an-Indian engineer, however this was based on a design with no beam except in one corner (concrete deck). My builder had experience of this concrete deck, having built a car showroom, and he wanted have miniature beams along all the external walls, and in from the columns (floor is 18cm thick, per spec, and walls 30cm wide, so these 'beams' are 12cm x 24cm wide (to allow for 3cm of cover) 2 * 16mm at top and at bottom

Electrical conduit shoved in randomly in wall cavity (no wiring). I am going to add a few more tomorrow.

'Beam' inside floor, running from columnPipe here I think doesn't actually need to be here. Basically all bathrooms/toilets have floor drains here. But I didn't want a pipe inside the slab, so this is above the slab level. Builder said we could raise the floor. I wonder if it's just not easier to do without a floor drain. However, it's usually for bathroom floors here to be a sopping wet mess. This however is just a secondary bathroom, so maybe it's ok?

Electrial conduit for terrace lighting running through slab (no ceiling outside!). Note also rebar chair thingies, as well as cement donuts. The conduit I think was rather cheap & nasty (10,000 IDR for 4 metres), and it appears not to be snug in the junction boxes (both are nominal 5/8", but the last minute-purchased conduit was not from our usual place, which was out of stock due to Ramadan, so it is not a match with the better quality junction boxes.) Plan is that the junction boxes are flush with the floor, so they are not covered with cement, and the wires can be inserted 'later'.

I have to provide food, apparently, for the concreting workers tomorrow. We still have not done a slump test, I had to attend to something just before 5pm, and when I came back at 5:30, the builders had gone home. So at 7am we need to start again. The rented (I have a smaller 150l) mixer is a 1 sack machine, so they want to dump a whole sack in.

There are many "mixtures" 1,1,3 .......1,2,3.....1,3,3. It doesn't go by weight, it goes by volume. Either way you'll be short on aggregate in the end. Do not let them dump in a bag of cement, it should be a bucket, then the bucket of sand etc etc etc, that way you get the correct mix for the job. You can't let them shovel it in and you can't let them just guess. It should have a set mixture, which might change for different parts of the structure where greater strength is needed.But your Indian rent an engineer should have designated that on the plans.

We just finished around 1am. We had 13 people working. The rented cement mixer had some loose bolts (equipment here is never in good condition). We had around 30 buckets split. The concrete seemed to harden pretty quickly - you could walk on it in an hour or so - but it remains to be seen if it has any voids.

We used 138 sacks of cement, slightly more than we had initiallly ordered. We also had to find some more sand at 6pm.

About half way through I noticed that the concrete wasn't flat at all, and that rebar was protruding in one place. Furthermore, the slab wasn't flat in any single direction (they just have a small hand held tool about a foot wide to flatten the pour). At that point they didn't have any builder's lines, so they hastily added some. Next time I will insist on many more.

The concrete mixing needed a lot of babysitting, although the basic process was ok with lots of buckets lined up, when some buckets broke they recalculated wrongly, and started putting too much sand in. Also I was making concrete with low slump at the bottom, and then I found out the people pouring it at the top were adding water to it. Also they would tend to scrape bits up off the floor, of dubious quality. Clearly readymix is a much better option, if available.

We just poured some more concrete. (Second time since the above). Now I just pay a fixed price and a gang of people come from the city and do it at a fixed price, no hidden costs. They bring with them their cement mixer, concrete lift , buckets and wheelbarrow. This works out better than using gangs of people on ladders and buckets, as they work about three to four times faster than the local slacker workers. We just poured around 25 cubic metres.

Unfortunately the concrete workers were adding 9 buckets of sand to a mix rather than the 7 I told them to. So we used less cement (205 instead of 250) than I had calculated, although as I don't trust the local steel, the workers, and so on, I tend to grossly overspecify the mix. Also I am now using a higher quantity of plasticizer (around 1%, which works out around half the cost of the cement), to try and get the water content down, as they really love their concrete sloppy. The The workers don't really care what they are producing, as soon as the last bucket of sand/gravel goes into the mixer, it gets dumped into the scoop to get hoisted up the top. (I think they probably should wait 30 seconds, although the total mixing time is I guess around 60 seconds, which is sufficient as the mixer is pretty high speed, that's from start to finish and the material goes in a bucket at a time)

I also noticed after pouring the slab for our kitchen that it was several centimetres lower than the rest of the building. I asked why this was, apparently this because kitchens have lots of water on the floor in Indonesia (they haven't thought of sinks, apparently), so my builder had decided I would benefit from a lower floor, which is a nice trip hazard. Will need to remedy that later. I have covered the new slab with black plastic, and we have had lots of rain, so that keeps things nice and cool.

Kitchen slab detailing:

I also am trying to retrofit hot water to an existing room. I purchased a Japanese gas water heater (10 kW or 5l/s rated @ 25C above input, which is plenty as the input is at least 20C) and mixer tap. The people in the building supermarket said I should install it inside the bathroom, and basically said to add a T fitting to the existing cold water pipe, and then connect one end of the mixer to that, and one end to the water heater, which then goes back to the other input on the mixer tap via an elbow bracket fixed to the wall. So they sold me some expensive Japanese hot water hose and t-fitting. However when I got the thing home I decided I didn't really want a gas bottle and heater inside the bathroom, so am going to try installing it on the outside wall (further burglar proofing is necessary for gas bottle, regulator, and water heater). I am not 100% convinced that the automatic ignition (it uses D batteries) is reliable enough to leave things outside, but still need to test.

Latest team of concrete pourers arrived on Saturday and we poured around 50 cubic metres at around 14 metres above street level.

They did a really terrible job, they were Batak (local tribe) who have a reputation for being stubborn.

They basically ignored whatever I told them and did what they wanted.

They also ripped me off on a hiring a vibrator.

I was expecting something like this:[img]http://www.mesinkomplit.com/wp-content/uploads/2014/09/External-Concrete-Vibrator.png[/img]however they delivered something like this

Which is actually not a bad product, just I wasn't aware it existed. The cost of one is only around 4000THB (local equivalent) with Yamaha engine, a little more with Honda. I was charged around 2/3 of the cost to buy to rent one.

I would say if starting again that buying this machine is mandatory for any kind of concrete work. The other thing I would change is to use better plywood, which is stronger (for vibration) and more reusable. The cheap plywood is around 250THB for 3 square metres/9mm thickness, and the good plywood (which is black rather than brown) is much thicker and costs at least twice that.

Anyway the concrete workers use a diesel-powered lift erected between a temporary rickety metal frame. The problem is that the process of lifting up the concrete and the vibrations suffered from the lift causes it to separate and become less flowable, so they add vast quantities of water. Ideally water content should be less than half the cement content (so 20 litres water for 40kg cement), and this includes the existing moisture from the sand (which may be wet), so actual added water content is less than 40% of the cement. Normal practice here is more like 100%.

As you can see from this graph

the strength at that ratio is almost non-existent. (For normal purposes you'd want >20 MPa strength)

We used A LOT of cement (520 sacks), and also water-reducing superplasticiser, both of which did reduce the amount of water, but with proper working practices we could likely have achieved more like 50 MPa with the given constituents, whereas we are lucky if we got 20.

Basically the constituents of the concrete get thrown (a bucket at a time) into the mixer, they add too much water and then they dump it in the lift. The problem with this is there is not enough mixing time. So you need to specify a minimum mixing time OR use two mixers, it depends on the volume. I think we got around 200kg in a mixer so 1 cubic is 12 loads. So for 50 cubic metres around 600 loads, I think you need to count maybe 1 minute to add the ingredients to the mixer PLUS the 1 minute mixing time making 2 minutes. This means a total of 20 hours, or realistically more with breaks etc. I would say one set of equipment is probably reasonable for 20 cubic metres of concrete, given that we finished around 3am and the standard of workmanship drops as the night drags on.

The other problem is that there is too much water because when the concrete reaches the top of the lift it is somewhat compacted by the vibration of the lift, and what they want is to dump the concrete on an inclined piece of plywood at the top and from there have it slide by itself into a wheelbarrow waiting below. (You can have someone in the plywood box helping the concrete down but anything other than a sort of diarrhea consistency means that they have to work pushing it down.) The problem with this is that properly mixed concrete does not flow (although superplasticiser does help here) so the concrete is far too wet. An explicit requirement for several people to be tasked with pushing the concrete down might help, but it would probably be better to have the concrete dumped directly into a large wheelbarrow at the bottom so the wheelbarrow can be brought directly to where it's needed, without the intermediate step of the bucket.

The fundamental problem is that the people working in the field literally have no understanding about the properties of concrete and they don't care either. So there is not really any correlation between how they work and what makes a strong concrete.

Latest team of concrete pourers arrived on Saturday and we poured around 50 cubic metres at around 14 metres above street level.

They did a really terrible job, they were Batak (local tribe) who have a reputation for being stubborn.

They basically ignored whatever I told them and did what they wanted.

They also ripped me off on a hiring a vibrator.

I was expecting something like this:[img]http://www.mesinkomplit.com/wp-content/uploads/2014/09/External-Concrete-Vibrator.png[/img]however they delivered something like this

Which is actually not a bad product, just I wasn't aware it existed. The cost of one is only around 4000THB (local equivalent) with Yamaha engine, a little more with Honda. I was charged around 2/3 of the cost to buy to rent one.

I would say if starting again that buying this machine is mandatory for any kind of concrete work. The other thing I would change is to use better plywood, which is stronger (for vibration) and more reusable. The cheap plywood is around 250THB for 3 square metres/9mm thickness, and the good plywood (which is black rather than brown) is much thicker and costs at least twice that.

Anyway the concrete workers use a diesel-powered lift erected between a temporary rickety metal frame. The problem is that the process of lifting up the concrete and the vibrations suffered from the lift causes it to separate and become less flowable, so they add vast quantities of water. Ideally water content should be less than half the cement content (so 20 litres water for 40kg cement), and this includes the existing moisture from the sand (which may be wet), so actual added water content is less than 40% of the cement. Normal practice here is more like 100%.

As you can see from this graph

the strength at that ratio is almost non-existent. (For normal purposes you'd want >20 MPa strength)

We used A LOT of cement (520 sacks), and also water-reducing superplasticiser, both of which did reduce the amount of water, but with proper working practices we could likely have achieved more like 50 MPa with the given constituents, whereas we are lucky if we got 20.

Basically the constituents of the concrete get thrown (a bucket at a time) into the mixer, they add too much water and then they dump it in the lift. The problem with this is there is not enough mixing time. So you need to specify a minimum mixing time OR use two mixers, it depends on the volume. I think we got around 200kg in a mixer so 1 cubic is 12 loads. So for 50 cubic metres around 600 loads, I think you need to count maybe 1 minute to add the ingredients to the mixer PLUS the 1 minute mixing time making 2 minutes. This means a total of 20 hours, or realistically more with breaks etc. I would say one set of equipment is probably reasonable for 20 cubic metres of concrete, given that we finished around 3am and the standard of workmanship drops as the night drags on.

The other problem is that there is too much water because when the concrete reaches the top of the lift it is somewhat compacted by the vibration of the lift, and what they want is to dump the concrete on an inclined piece of plywood at the top and from there have it slide by itself into a wheelbarrow waiting below. (You can have someone in the plywood box helping the concrete down but anything other than a sort of diarrhea consistency means that they have to work pushing it down.) The problem with this is that properly mixed concrete does not flow (although superplasticiser does help here) so the concrete is far too wet. An explicit requirement for several people to be tasked with pushing the concrete down might help, but it would probably be better to have the concrete dumped directly into a large wheelbarrow at the bottom so the wheelbarrow can be brought directly to where it's needed, without the intermediate step of the bucket.

The fundamental problem is that the people working in the field literally have no understanding about the properties of concrete and they don't care either. So there is not really any correlation between how they work and what makes a strong concrete.